(1) Field of the Invention
Plastics with high flexural modulus, high tensile and compressive strength, and capable of being shaped into useful forms are water-insoluble and hydrophobic to a high degree, so that they are poorly wetted by water. Thus, water tends to form beads on the surface of an aircraft enclosure made of poly(methyl methacrylate), for example. Light transmitted through such a spottily wetted enclosure tends to be refracted and distorted by the multitude of water-droplet lenses on the plastic surface, and inefficient windshield wipers are called into play to improve the acuity of vision possible through such an enclosure. Enclosures whose surfaces exposed to the weather are coated with polysaccharides according to this invention, are uniformly wetted by water which does not "bead" on the surface but forms a continuous sheet of liquid causing little or no distortion of transmitted light.
Similarly, spectacle lenses made from the plastics commonly known as polycarbonate are hydrophobic and present a distorted view when wet with droplets of water. The golfer or pedestrian wearing such spectacles in a rain shower will suffer from impaired vision. In contrast, polycarbonate lenses prepared according to this invention are wetted uniformly by water and continue to provide good vision.
Similarly, body implants, such as plastic lenses for the eye and orthopedic joints and other bone replacements, often present problems with subnormal lubrication as the body recovers from the trauma of disease and its corrective treatment. It is not uncommon for the patient to endure pain and limited use of the affected part for lengthy rehabilitation periods. We find much improved results with products made according to the teachings of this invention, wherein the parts to be implanted are permanently coated with hydrophilic and highly lubricious polysaccharides closely related chemically and in physical character to materials normally present and acting as lubricants and humectants in the body domain.
Still another illustration of the application of this invention is in the manufacture and use of permanently slippery catheters. Plastic catheters with the grafted or crosslinked polysaccharide coatings, stored in aqueous medium such as isotonic solution, can be inserted into body orifices as necessary with remarkable ease and absence of pain. Similarly, vascular connectors to be left in the artery or vein for long periods while extracorporeal blood treatments are made can be less painfully inserted and often show decreased thrombosis formation when fabricated by the methods of this invention. This should contribute significantly to much more satisfactory long-term acceptance of the plastic intravascularly.
A basic feature of the invention which underlies all of these and other applications is that the surfaces of the treated plastic objects display wettability and lubricity uncharacteristic of such materials. Normally, materials showing such hydrophilic properties, such as gelatin, are easily distorted, have little or no strength, and show low modulus in flexure. The products of this invention display permanently the desirable surface characteristics of soft materials and at the smae time have the strength and modulus of underlying structural plastics.
A second and basic distinguishing feature, related to the first, is the heterogeneous, laminar structure and composition of the products of this invention, comprising a load-bearing structural plastic whose surface is coated by an insoluble polysaccharide film intimately bound by high specific adhesion and/or by chemical bonds to the substrate. A cross-section of the articles of this invention, examined analytically from their outermost boundary into the bulk interior, would show a sharp transition from insoluble polysaccharide, thousandths of an inch thick, through a few mils of a tie-coat of markedly different composition, and finally into the main supporting plastic, such as polycarbonate, polymethyl methacrylate, polystyrene, polyformaldehyde, and the like. This chemical and physical heterogeneity is an essential feature of the new compositions of value.
(2) Description of the Prior Art
It is well known to use polysaccharides such as hyaluronic acid and chondroitin sulfate in ophthalmic procedures and orthopedic surgery, to provide temporary lubrication of ocular or bone surfaces and interfaces. The desirable effects are only temporary, and repeating the treatment may not be possible after wound closure. There may also be unfavorable consequences of these uses, such as indications of glaucoma in some cases after use of preparations containing polysaccharides in ophthalmic surgery.
Ivani (U.S. Pat. No. 4,365,050) teaches the use of polysaccharides as components of mixtures to function as "artificial tears" to be dropped into the eye frequently, in contact lens cleaning solutions, and in anti-inflammatory ointments. He claims polymeric compositions formed by reacting polysaccharides with polymerizable monomers, silicone, pyrrolidone, and collagen. As shown clearly by his examples and disclosures, his polymeric compositions are prepared by conventional techniques such as mastication; emulsion, suspension and solution polymerization; and bulk polymerization in molds.
None of Ivani's polymer products anticipate the uniquely permanent, laminar structures of this present invention. Ivani's ophthalmic polymers, for example, are intended not to leave a residue of film on the contact lenses (Column 10, lines 62-63). The very preparative techniques employed by Ivani are unable to produce the laminar polymeric articles of this invention having hydrophilic outer surface. Ivani's only examples of forming his polymeric compositions show the grafting of methacrylic acid or MMA onto Chitin Flake, in which any laminar structure that might occur by chance would have the insoluble polysaccharide covered with acrylic polymer, a reversal of our invention, defeating its very purposes.